In general, if a mechanical shock such as vibration occurs from the outside in the case of a bolt and a nut which are combined in the form of a screw, the nut or bolt is loosened due to clearance existing between threads of the bolt and nut, and thus a structure, device or facility combined by the bolt and nut becomes loose, to resultantly cause an accident to occur. In particular, to prevent bolts and nuts from being loosened in devices or facilities which are installed in places where vibrations are severely generated, such as vessel engines, piping facilities, steel plants, power generation facilities, railroads, and bridges, is to avoid an accident from occurring. Accordingly, various devices which prevent nuts and bolts from being loosened are being developed as well as are already developed.
One of them includes an anti-loose means such as a coil spring in the inside of the nut. Such a conventional anti-loose lock nut is illustrated in FIG. 1. As illustrated, a coil spring 11 is installed around the inner circumferential surface of a nut 10. A bent portion 30 which is protruded to the outside is formed at the end of the coil spring 11. The bent portion 30 is fit-coupled with an insertion groove 31 which is formed on the bottom of the nut 10. Thus, the coil spring 11 is fixed not to rotate around the inner circumferential surface of the nut 10. In addition, an extended diameter portion 32 whose inner diameter is wider than the thread 13 is formed in the inner circumferential surface of the nut 10. A spiral groove 33 which guides the coil spring 11 is formed in the extended diameter portion 32. Thus, the coil spring 11 is inserted into the spiral groove 33 to then be fixed. The coil spring 11 is fabricated by winding an elastic wire whose cross-section is a regularly circular form in a spiral direction. A wire whose diameter equals or smaller than the thread pitch of the bolt 12 is used as the elastic wire of FIG. 1.
As described above, the bolt 12 is fixed by the coil spring 11, and the threads 14 of the bolt 12 closely contact and are tightly tied with the gully portions of the coil spring 11. Accordingly, the bolt 12 is not loosened from the nut 10. Thus, if the nut 10 circumvolves by a stronger force than intensity of the coil spring 11, in order to unlock the bolt 12 and the nut 10, the bent portion 30 of the coil spring 11 is cut and untied.
However, since the spiral groove 33 is formed in the extended diameter portion 32, and thus the coil spring 11 is inserted into and fixed to the spiral groove 33, it is difficult to match the bolt 12 and the coil spring 11 in the case that the start point of the thread 14 of the bolt 12 does not equal to the start point of the windings of the coil spring 11. In particular, carbon steel of high carbon content or alloy steel including chrome is mainly used as the elastic wire. Since spring steel such as carbon steel of high carbon content or alloy steel including chrome has a higher intensity than general steel which is used as general bolts, the thread 14 of the bolt 12 can be worn out or damaged before the bolt 12 is rotated into and matches the coil spring 11. Accordingly, if the bolt 12 and the coil spring 11 are fitted with each other by force at the state where the bolt 12 and the coil spring 11 do not match, the screw coupling between the bolt 12 and the coil spring 14 becomes unstable. As a result, structures or facilities which are combined with the bolts and nuts may be defective to accordingly cause dangerous accidents to occur.
In addition, since the cross-sectional area of the coil spring 11 is fabricated to have a regularly circular type cross-section having the same or smaller diameter as that of the pitch of the bolt 12, the cross-sectional area of the coil spring 11 is limited. Accordingly, there is a limit to heighten elasticity and intensity of the coil spring 11. Further, since the coil spring 11 has no sufficient force for tightening the bolt 12, there is a shortcoming that the bolt 12 is loosened by a small amount of force. In addition, it is preferable that the intensity with which the bent portion 30 is cut is varied according to the use place in the case of the anti-loose lock nut, but since the cross-sectional area of the coil spring 11 is limited, it is difficult to control the intensity of the bent portion 30 in various amounts. As a result, there is a problem that the bent portion 30 is unfolded or cut, to thereby cause a function of holding the bolt 12 to be lost.